1. Technical Field
This disclosure relates generally to acoustic panels and manufacturing methods thereof.
2. Background Information
Various types and configurations of acoustic panels are known in the art for applications such as aircraft nacelles and include single degree of freedom (SDoF) acoustic panels and double (or dual) degree of freedom (DDoF) acoustic panels.
A SDoF acoustic panel may include a core material such as honeycomb core bonded between a pair of exterior panel skins, typically referred to as a top skin (which faces the noise source to be attenuated) and a bottom skin. The top skin is perforated to allow noise waves into cavities formed by the core for attenuation. Each cavity formed by the core is a resonating chamber and reflects back out of the top skin perforations a corresponding sound wave which is intended to be out of phase with the incoming sound wave so that they destructively interfere. Such a SDoF acoustic panel is effective in attenuating only a specific target noise frequency, or a limited, narrow band of frequencies around the target frequency.
A DDoF acoustic panel is distinguished from a SDoF acoustic panel in that the DDoF acoustic panel has a different core structure which forms two different resonating chambers to attenuate noise around two target frequency bands for broader band noise attenuation. One type of DDoF acoustic panel is essentially a SDoF acoustic panel with an additional skin called a septum positioned between and generally parallel to the top skin and back skin to divide the cavity formed by the core walls, top skin and back skin into two distinct cavities. The septum is perforated so that sound waves can enter through the perforations in the top skin, travel through the first cavity, through the perforations in the septum, and into the second cavity. The target frequencies, which are essentially the resonant frequencies of the two cavities or resonating chambers, are a function of the cavity geometry and other factors, and is typically adjusted by selecting the overall depth between the top skin and back skin, and the depth of the septum.
Two examples of DDoF acoustic panels are shown in FIGS. 1 and 2. In FIG. 1, the DDoF panel is formed by a single core structure extending continuously between the top skin and bottom skin, and discrete septums are individually placed in the cavities formed by the core. In FIG. 2, the DDoF panel is formed by two independent layers of core structure, with a continuous septum layer separating them. Here, the two independent layers of core structure are not necessarily aligned or registered with one another such that the cavities fainted in each layer may overlap to at least some extent. In each case, the core structure is typically bonded with an adhesive at each joint it forms with the top skin, bottom skin and septum.
In the case of a DDoF acoustic panel constructed in the manner shown in FIG. 1, the septum is typically constructed of a wire mesh or tight screen made of metal or plastic, and the discrete septum pieces are individually bonded in place inside of each cavity formed by the core. The manufacturing costs for this style of construction can be very high, based in part on the difficulty of shaping and positioning the many small, discrete pieces of septum in each cavity.
In the case of a DDoF acoustic panel constructed in the manner shown in FIG. 2, the septum may be constructed from a continuous sheet of wire mesh material or a tight screen made of metal or plastic, or a solid metal or plastic skin with discrete perforations formed therein, or other configurations, but must be permeable in some fashion so that the air waves can travel through. With such a configuration, the bonding strength between walls of the first and the second core structures and the septum may be relatively weak since portions of the walls overlap pores or perforations in the septum, reducing the bonding strength. Also, if using an adhesive bonding process to connect the two core structures to the septum, care must be taken to avoid the adhesive wicking onto the septum and closing the pores or perforations.
There is a need in the art for improved acoustic panels and method for manufacturing such improved acoustic panels.